The lack of satisfactory brightness is often a concern for electrophoretic display devices. Total internal reflection inevitably would occur with electrophoretic display devices because a display device usually has components of a high refractive index. Due to the component having a higher refractive index (e.g., about 1.5) than the air (which has a refractive index of about 1) surrounding the display panel, some of the scattering light from the display panel may reflect back to the display device by total internal reflection. This total internal reflection phenomenon may result in a loss of about 30-50% of the scattering light, thus causing reduction in brightness of the display device.
The Lambertian reflectance which is part of the nature of electrophoretic displays is beneficial for certain display applications. This is so because it allows viewing of a display panel at all angles, with almost the same brightness. However, the Lambertian reflectance is not important for all display applications. For example, for an e-reader, the viewers would tend to view the e-reader display within a certain angle, typically at normal angle of incidence to the display. In other words, the off-axis brightness of an e-reader is less important than the on-axis brightness. Therefore, it might be beneficial in such a case to trade the off-axis brightness for improved on-axis brightness.